Phased array acoustic imaging system

ABSTRACT

A phased array acoustic imaging system including an array of electromechanical transducers, a dedicated signal processing channel for each transducer and electronic control and delay circuits for them, transmits to and receives ultrasonic energy from a body in a controlled, steered and focused fashion. The electronic delay circuits use a unique cyclically symmetrical arrangement of identical delay cells, each with an electronically variable delay element and each dedicated to a receive signal derived from at least one transducer, to form a summing delay line. The individual delay cells are switched under microprocessor control for efficient combined linear-sector scanning or linear or sector scanning alone. The system includes means to achieve dynamic focusing and dynamic apodization in conjunction with the delay cell architecture.

BACKGROUND OF THE INVENTION

Recent improvements in the use of ultrasonic energy to illuminate theinternal organs of the human body have included "realtime" imaging,which enables visualization of the motion of the organs, and"gray-scale" imaging, which gives diagnostic information concerning thetexture of the soft tissue in the body. More recently, ultrasonicimaging using arrays of transducers with appropriate control electronicshas proven successful because of transducer reliability, flexibility andcapability for steering and focusing ultrasonic energy, and has improvedresolution over a certain depth of field, particularly near thetransducer.

Improvement beyond the present state of the art requires largertransducer arrays with more elements, but this requires correspondinglymore circuitry. In order to produce such systems economically,innovations must be made in system architecture to permit flexiblesystem operation which makes extensive use of symmetry to minimizededicated hardware without sacrificing performance, consistent with highresolution imaging.

An example of previous array technology is described in U.S. Pat. No.4,140,022 granted Feb. 20, 1979 to Samuel H. Maslak and assigned toHewlett-Packard Company, where extensive use of tap selectors isillustrated. In the there-described system, the number of analogswitches required for tap selection depends upon the product of thenumber of input signals, which is equal to or proportional to the numberof active transducer elements, and the number of taps in the summingdelay line configuration. In order to improve the performance of such asystem, by doubling the number of transducer elements, for example, thenumber of analog switches must increase by a factor of four toaccommodate the longer delay line needed for the correspondingimprovement in performance. These analog switches contributesubstantially to system cost because they should be low-noise,broad-bandwidth components. It is, therefore, desirable to eliminate theneed for tap selection completely. Furthermore, the foregoing patentedsystem provides a sector scan but does not provide for linear scanning.

Another example of the prior art, U.S. Pat. No. 4,005,382 granted onJan. 25, 1977 to William L. Beaver and assigned to Varian Associates,does avoid tap selection. But the described system is unable to providethe non-monotonic delay profiles required to achieve significantfocusing of the ultrasonic wave at angles at or near the normal to thetransducer. That patented architecture requires "zero delay" electricalconnections among the outputs of adjacent delay elements, which createssevere technological difficulties in a high resolution imaging systemwith many simultaneously active transducer elements. That patentedarchitecture also lacks the cyclical symmetry needed for a linearscanning format, wherein sequential acoustic lines are activated, eachnormal to the transducer but displaced by an amount along the transducerarray which might be equal to the transducer element-to-element spacing.

SUMMARY OF THE INVENTION

The principal object of this invention is to provide an acoustic imagingsystem with a very flexible architecture which can accommodate both alinear and a sector scanning format for an array of a very large numberof active electromechanical transducer elements, such as 128, in aneconomical fashion. The basis of this architecture is a delay cell whichcontains an electronically variable delay element and which is dedicatedto a receive signal derived from at least one transducer. Thearchitecture comprises a cyclically continuous structure, so that eachdelay cell, including the end cells, have an adjacent cell to the rightand to the left. The individual delay cells are configured to achieve adesired delay profile for each scan line under digital control.

Each delay cell contains an electronically variable delay element,control electronics, summers, and solid state switches to sumselectively (1) the derived transducer receive signal with other signalsinput to the delay cell including (2) the output of the previous delaycell, and/or (3) the output of the next delay cell, and/or (4) theoutput from one arbitrary other delay cell. The output of each delaycell may be selectively routed to one of several different places, i.e.(1) to the input of the previous delay cell, or (2) to the input of thenext delay cell, or it may be "out-selected" for (3) video processing or(4) for "in-selection" to the input of one selectable other delay cell.

Such an architecture provides means to structure a summing delay line soas to achieve a non-monotonic delay profile which is necessary forwell-focused acoustic lines especially when steered to within a fewdegrees, such as up to 15° or so, of normal to the face of thetransducer in the sector scanning mode. In the linear scanning mode,such architecture provides means to generate an acoustic focus, uponreception, along parallel acoustic lines, all of which are at a fixedangle which might be normal to the transducer array, in such a way thateven the acoustic lines centered near the last transducer of the arrayutilizes the entire summing delay line. This capability results from thecyclic symmetry of the delay cell architecture, and avoids the need todouble the number of delay cells in order to achieve comparableperformance.

It is another object of this invention in combination with the delaycell architecture to include heterodyning or mixing of each transducerreceive signal with one of a plurality of individually phased clocksprior to delay so as to achieve phase coherence with one or more othertransducer receive signals processed in a similar way. This allows theset of delay cells to be coarsely quantized.

It also is an object of this invention to sum such heterodyned and phaseadjusted transducer receive signals in phase coherent triplets,quadruples or, in the preferred embodiment, in phase coherent pairs,prior to connection to delay. This achieves a substantial reduction inthe number of delay cells required in a particular system.

Another object of this invention is to provide means to dynamically varythe clock phase mixed with each receive signal at various times duringreception in order to maintain phase coherence of all active receivechannels to within some predetermined amount which might be less than45°.

It is also an object of this invention to dynamically update theamplitude weighting of the receive signal from each transducer,including a weighting of zero, at various times throughout reception inorder to improve focusing and sidelobe performance.

One other object of this invention is to include static amplitudeweighting means, including a weighting of zero, for the linear scanningmode to generate acoustic scan lines up to and including one centered onthe last transducer element in the array.

Another object of this invention is to maintain an approximatelyconstant ratio of focal range to size of active aperture throughout atleast a portion of the field of view.

It is also an object of this invention to provide an imaging system thatoperates in a sector mode only, or in a linear mode only, or in acombined linear/sector mode.

Other objects and advantages will be apparent from a consideration ofthe following description of a preferred embodiment of the invention inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the system architecture;

FIG. 2a graphically depicts the delay profile for a scan line normal toa planar transducer array at the center of the array;

FIG. 2b is a schematic diagram of the delay cell arrangement for thedelay profile shown in FIG. 2a;

FIG. 3a graphically depicts the delay profile for a scan line normal toa planar transducer array centered between a pair of transducers 48,49of the array;

FIG. 3b is a schematic diagram of the delay cell arrangement for thedelay profile shown in FIG. 3a;

FIG. 3c graphically depicts the delay profile for a scan line normal tothe array at one end of the pairs of transducers of FIG. 3a;

FIG. 4a graphically depicts the delay profile for a scan line where thesteering angle ranges from zero to about 15° from normal to a planartransducer array;

FIG. 4b is a schematic diagram of the delay cell arrangement for thedelay profile shown in FIG. 4a;

FIG. 5a graphically depicts the delay profile for a scan line where thesteering angle is greater than about 15° from normal to a planartransducer array; and

FIG. 5b is a schematic diagram of the delay cell arrangement for theprofile shown in FIG. 5a.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates the system architecture in a preferredembodiment that employs phase coherent or phased pairs of transducersignals. For each acoustic scan line the electromechanical transducerarray launches an acoustic pressure wave which is electronically steeredand focused. Transducers 10, 10' of one phased pair are activated by adrive pulse from their associated transmit drivers 20,20' which havebeen amplitude weighted by values stored in transmit apodization memory22. The radial direction of a transmitted acoustic pressure wave fromthe array for sector scanning is determined by the relative times atwhich each transducer of the array is pulsed.

A signal which has appropriate time delay is generated in the transmitdelay cell 24. These time delay values, one for each active transducerelement, which cause the transmitted acoustic beam to be steered andfocused, are stored in transmit delay memory 26. The time delayed signalgenerated in each transmit delay cell is supplied to each transmitdriver 20, which then activates each transducer element 10.

Transducers 10,10' are two adjacent ones of a planar array of onehundred twenty-eight identical electromechanical transducers, forexample numbered 0 through 127 as shown.

At the completion of transmission of an acoustic wave each transducer 0through 127, such as phased pair 10,10' begins to receive acoustic echosfrom specular and Rayleigh reflectors in the body. Each transducerconverts the received acoustic echo into an electric receive signalwhich is processed in a separate receive channel dedicated to eachtransducer including, for the phased pair 10,10', protection circuit28,28' to isolate the channel from transmit pulses, a variable gainamplifier 30,30' and a receive apodization amplifier 32,32' whichamplitude weights each receive signal to values stored in receiveapodization memory 40. Scanner gain driver 34 in accord with valuesstored in gain control memory 36 supplies dynamically up-dated bias andgain signals to variable gain amplifier 30,30'. Amplitude weighting isindependently selected for each channel by receive apodizers 38connected to apodizer amplifiers 32,32' in accord with values stored inreceive apodization memory 40. The weighting may be dynamically changedduring reception.

The receive apodizer amplifier output, which has most of its energy nearits associated transducer center frequency, may be mixed in mixer 42,42'with one of a plurality of individually phased clock signals, such ascos (ω_(LO) t+Ω₀) and cos (ω_(LO) t+Ω₁) generated from a local referenceoscillator by mixer clock generator 44 in accord with values stored inphase control memory 46. The particular clock phase angle is selected sothat output 48 of mixer 42 has phase coherence with the output 48' ofmixer 42' in the adjacent channel which is dedicated to transducer 10'.The outputs of mixers 42,42' being phase coherent to within reasonableaccuracy, such as 45° or less, are summed in summing amplifier 50,creating a phased intermediate frequency signal 52 such as P_(i) (0,t)for the first phased pair of transducers 10,10' (transducers 0,1) up toP_(i) (63,t) for the last phased pair of transducers (transducers126,127) in the array. If the individual receive channels are notgrouped, such as in phased pairs, P_(i) (0,t) may be simply the mixeroutput 48 from a single non-grouped receive channel.

The several memories are accessed and values updated dynamically bydigital control means 54 which may include a microprocessor. The dynamicphase-shifting signal processing in a dedicated channel for eachtransducer is of the type more fully described in U.S. Pat. No.4,140,022. However, instead of using the there-described tap selectorand tapped master delay line, the present invention constructs a summingdelay line 56 from a set of symmetrical delay cells 58, one for eachphased intermediate frequency signal, such as 52, derived from a phasedpair or other grouping of transducers in their corresponding receivesignal processing channels. The phased intermediate frequency signalsmay also be derived from phase coherent transducer triplets orquadruples in order to allow the delay cells to be more coarselyquantized and to further reduce the number of delay cells necessary fora particular system. Alternatively, the phased intermediate frequencysignals may be derived from phase coherent transducer singlets to allowmore finely quantized delay cells at the expense of an increased numberof delay cells.

Each delay cell 58 includes an input summing means such as summingamplifier 70 for adding phased intermediate frequency signal 52, P_(i)(0,t), and any combination of three other input signals. Each delay cell58 also has a delay element 72 which may have one or more selectabledelays. In the described embodiment, there are two choices, no delayother than the normal electronic delay and a preselected finite delay,such as 230 ns, depending upon the position of electronic delay switchmeans 74.

The other delay cell inputs to summing amplifier 70 of delay cell 58₀,in addition to (1) the phased intermediate frequency signal 52, P_(i)(0,t), include (2) input A_(i) (0,t) from the output of the previousdelay cell 58₆₃ in the summing delay line 56; (3) input B_(i) (0,t) fromthe output of the next delay cell 58₁ ; and (4) an inselected delay celloutput, INSEL(0,t), which can come from the output of a selected delaycell. As shown in FIG. 1 for delay cell 58₀ the previous delay cell 58₆₃is physically at the other end of the delay line 56, but electrically itis adjacent.

Each delay cell 58 has three selectable output paths depending upon theposition of electronic delay cell output switch means 76. The selectableoutputs of delay cell 58₀ are A_(o) (0,t), B_(o) (0,t) and OUTSEL(0,t).As shown in FIG. 1, B_(o) (0,t) and A_(o) (0,t) go, respectively, to theinputs of the previous 58₆₃ and the next 58₁, delay cell. OUTSEL(0,t)enables the output of the delay cell to be routed to an associatedoutselect summing amplifier 78 where it may then be routed by acorresponding electronic distribution switch 80 to the inselect summingamplifier 82 and inselect switch means 84 or, when delay processing iscomplete, to the system intermediate frequency summing point at IFsumming amplifier 86 for final video detection, other processing anddisplay at 88. The associated outselect summing amplifier 78 output alsomay be decoupled from the signal path at 90.

In the preferred embodiment, eight outselect summing amplifiers 78 areemployed. The OUTSEL outputs of eight adjacent delay cells are connectedto the input of one outselect summing amplifier 78. The output of eachoutselect summing amplifier is independently routed by its associateddistribution switch 80, to one of (1) the IF summing amplifier 86 (2)the INSELECT summing amplifier 82, or (3) is decoupled from the signalpath at 90.

As the several intermediate frequency signals P_(i) (0,t) through P_(i)(63,t), such as 52, enter the delay line 56, they add to theintermediate frequency signals inserted from earlier delay cells in thesequence so that all the frequencies combine at the summing point, IFsumming amplifier 86, in overlapped relationship with substantial phasecoherence. The appropriate frequency band of the IF summing amplifier 86output signal is selected by a filter in video detection means 88 andthen processed for cathode ray tube or other display.

There is cyclical symmetry at the ends of the summing delay line 56.Input A_(i) (0,t) of the first summing amplifier 70 comes from theoutput A_(o) (63,t) of last delay cell 58₆₃. Also, input B_(i) (63,t) ofthe last delay cell 58₆₃ comes from the output B_(o) (0,t) of the firstdelay cell 58₀. In effect, the delay line is completely circular in itsarchitecture. This symmetry greatly simplifies the design by enablingthe same data which is used to control the signal flow to be reused on agreat number of different acoustic scan lines by merely rotating thedata by means of digital control means 54.

The summing delay line 56 configuration, under several representativeconditions, is illustrated graphically in FIGS. 2a, 3a, 3c, 4a and 5a.Each graphically shows a particular delay profile, where the verticalaxis corresponds to relative delay and the horizontal axis correspondsto phased pairs of transducers such as 10,10'. The delay profile shouldproperly be shown in quantized delay units, but for the sake of clarity,quantization effects have been ignored. In FIGS. 2b, 3b, 4b and 5b thecorresponding summing delay line schematic configurations are shown. Inthese figures, each delay cell 58 is indicated by input summing meansequivalent to summing amplifier 70 of FIG. 1 with the mixed intermediatefrequency signal from one phased pair, such as 52, typically added withthe output of an adjacent delay cell.

In particular, FIG. 2a illustrates a delay profile required for a scanline in the center of the array and normal to it originating betweentransducer phased pairs 31 and 32. FIG. 2b illustrates the configurationof the summing delay line 56 which implements the delay profile shown inFIG. 2a. The phased intermediate frequency signals P_(i) (31,t) andP_(i) (32,t) supply the input summing means 70 in the center delay cells58₃₁ and 58₃₂. Signals to the left, including P_(i) (31,t) to P_(i)(0,t) are steered leftwards in the delay line, successively adding delayin the series connected delay cells, whereas signals to the right,including P_(i) (32,t) to P_(i) (63,t) are steered in the oppositedirection. In this example, the entire aperture is utilized.

Delay cell output switch means 76 in end delay cell 58₀ routes summedand delayed signal OUTSEL(0,t) to associated summing amplifier 78. Soalso does switch 76 in end delay cell 58₆₃ route OUTSEL(63,t) to itsassociated summing amplifier 78. Both signals OUTSEL(0,t) andOUTSEL(6,t) then connect via associated distribution switches 80 to thesumming point at IF summing amplifier 86 in time-overlapped relationshipwith substantial phase coherence.

A linear scanning format with an active scan line normal to the arraybetween transducer phased pairs 48 and 49 is shown in FIG. 3a. Asdescribed above, transducer phased pairs equal to or less than pair 48steer to the left and pairs greater than 48 steer to the right. In thedelay line configuration shown, each phased intermediate signal fromP_(i) (17,t) to P_(i) (48,t) has successively more delay. P_(i) (17,t)requires no additional delay, so the output of delay cell 58₁₇ is routedby switches 76,78,80 to the IF summing amplifier 86. On the other sideof the array, signals P_(i) (49,t) to P_(i) (63,t) are steered to theright and, in fact, P_(i) (63,t) is also steered to the "right" by meansof the cyclic symmetry described earlier, which connects it to the inputsumming amplifier 70 of delay cell 58₀ to provide the additional delayof cells 58₀ to 58₁₆ as shown in FIG. 3b. Phased intermediate frequencysignals P_(i) (0,t) to P_(i) (16,t) are turned off by receive aperturecontrol means, such as apodizer amplifiers 32, in accord with apodizer38 and apodization memory 40 of FIG. 1 and the active aperture consistsonly of phased pairs 17 through 63.

This static aperture control can be used to de-activate successivephased pairs while simultaneously shifting the delay profile to theright in this cyclic fashion in order to successively translate scanlines down the array, consistent with a linear scanning format. In thelimiting case, the last scan line is placed over the last element in thearray. In this case, as shown in FIG. 3c, half of the array is active.Delay cell 58₃₂ is outselected to IF summing amplifier 86 and the phasedintermediate frequency signals P_(i) (0,t) to P_(i) (31,t) are apodizedoff.

FIG. 4a illustrates delay profiles required in the sector scanning modefor steering angles θ which are between zero and 15° or so. This is anon-monotonic delay profile with no particular symmetry. In the exampleshown, the maximum delay is assumed to be between transducer phasedpairs 36 and 37. For phased pairs less than or equal to 36, the delaypath progresses in the usual way to the left. The output of delay cell58₀ is routed to the IF summing amplifier 86. The delay required fortransducer pairs 37 to 63 is obtained by steering phased intermediatefrequency signals P_(i) (37,t) to P_(i) (63,t) rightward through theirrespective delay cells. The output of pair 63 is routed to the inselectline of delay cell 58₁₆ in order to use the required additional delayprovided by delay cells 58₁₆ to 58₀. The output of delay cell 58₀ isrouted to the IF summing amplifier 86.

The last example, FIGS. 5a and 5b, shows a delay line configuration fora typical off-axis case, where the steering angle θ is greater than 15°or so. In such a case, the maximum delay occurs at the end of the array.Therefore, each phased pair starting with 63 merely looks leftward allthe way to the first phased pair 0, which is then routed to the IFsumming amplifier 86.

The foregoing examples are used to illustrate the delay cellarchitecture in invention. Many variations are useful and possible whichmay be apparent to those skilled in the art, such as replacement ofdelay element 72 with several discrete delay elements in order to obtainsmaller quantization values.

One can eliminate outselect summing amplifier means 78, distributionswitch means 80, decoupling 90, and inselect summing amplifier 82 andswitch means 84 by connecting all the delay cell OUTSEL outputs directlyto the IF summing amplifier 86 in a linear scanning mode. A sectorscanning mode can be implemented by providing outselect summingamplifier means for the OUTSEL outputs for the outermost delay cells,58₀ and 58₆₃ only. One can also eliminate inselect means entirely fromthe architecture and still allow scanning in a combined linear/sectormode over the entire array and over all sector angles by addingadditional delay cells. The delay line can also operate with inselectmeans or outselect means or both wherein the cyclic architecture is notused. Other combinations of the number of outselect summing amplifierswith delay cell OUTSEL outputs are also possible with this architecture.

These and other modifications are within the scope of the inventiondefined in the following claims.

We claim:
 1. In an acoustic imaging system having means for repeatedlytransmitting an acoustic pressure wave into a body to be examined, anarray of transducers for transducing the acoustic echo that impinges oneach of them into a corresponding electrical receive signal, one of aplurality of substantially identical channels for processing the receivesignal from each transducer and an intermediate frequency summing pointfor combining, for each scan line, the processed receive signal fromeach of the transducers in the array, the improvement comprising:asumming delay line causing the processed receive signal for eachtransducer to arrive at the summing point in time overlappedrelationship including a plurality of delay cells forming the summingdelay line in selective sequence, each delay cell consisting of(a) delaycell input summing means for at least one of (1) a signal derived fromthe processed receive signal of at least one transducer, (2) aninselected signal, (3) the output of a first adjacent delay cell or (4)the output of a second adjacent delay cell to produce a delay cellsummed input signal, (b) delay cell output switch means, and (c) atleast one discrete delay element connecting the delay cell summed inputsignal to the delay cell output switch to form a delay cell output, saiddelay cell output switch selectively connecting the delay cell output toany one of (1) the input summing means of a first adjacent delay cell,or (2) the input summing means of a second adjacent delay cell or (3) anoutselection path; and a digital control means for coordinating theposition of the output switch means for each delay cell to configure thedelay line in a preselected manner for each particular scan line of saidtransducer array.
 2. The acoustic imaging system of claim 1 furthercomprising:inselect switch means for connecting an inselected signal toa selected one of said delay cell outputs; and wherein the digitalcontrol means coordinates the position of the output switch means foreach delay cell and said inselect switch means to configure the delayline in a preselected manner for each particular scan line of saidtransducer array.
 3. The acoustic imaging system of claim 1 furthercomprising:outselection distribution switch means for selectivelyconnecting the outselection paths of said delay cells to said summingpoint; and wherein the digital control means coordinates the position ofthe output switch means for each delay cell and said outselectiondistribution switch means to configure the delay line in a preselectedmanner for each particular scan line of said transducer array.
 4. Theacoustic imaging system of claim 1 further comprising:an inselectsumming amplifier; inselect switch means for connecting an inselectedsignal of one of said delay cells to an outselected signal at theinselect summing amplifier; outselection distribution switch means forselectively connecting the outselection paths of said delay cells to oneof said inselect summing amplifier or said summing point or fordecoupling it; and wherein the digital control means coordinates theposition of the output switch means for each delay cell, said inselectswitch means and said outselection distribution switch means toconfigure the delay line in a preselected manner for each particularscan line of said transducer array.
 5. The acoustic imaging system ofclaim 1 wherein the discrete delay may be normal electronic delay or adiscrete delay of greater preselected value.
 6. The acoustic imagingsystem of claims 1, 2, 3, 4 or 5 wherein the transducers are arranged ina planar linear array in side-by-side fashion.
 7. The acoustic imagingsystem of claims 1, 3, 4 or 5 wherein each scan line in a linear scanmode is at a predetermined substantially fixed angle to the normal tothe transducer array and its position is determined by said digitalcontrol means.
 8. The acoustic imaging system of claims 1, 2, 4 or 5wherein each scan line is at an angle with respect to a normal throughthe transducer array with a common point of intersection, where both theangle and the point of intersection are determined by said digitalcontrol means.
 9. The acoustic imaging system of claims 1, 2, 3, 4 or 5wherein each delay cell has more than one discrete delay element andfurther includesdelay switch means for selectively connecting the delaycell summed input signal to the delay cell output switch through one ormore of the discrete delay elements to form a delay cell output, andwherein said digital control means also coordinates the position of thedelay switch means for each delay cell.
 10. The acoustic imaging systemof claims 1, 2, 3, 4 or 5 wherein the digital control means achievescyclical symmetry among the delay cells by electrically connecting thefirst and last delay cells in said delay line so as to beindistinguishable from adjacent delay cells.
 11. The acoustic imagingsystem of claims 1, 2, 3, 4 or 5 further includingphase adjustment meansto achieve phase coherence between the receive signals of at least twoadjacent transducers to permit coarse quantization of the delay elementsin said plurality of delay cells.
 12. The acoustic imaging system ofclaim 11 further includingheterodyning means in conjunction with thephase adjustment means in order to derive a phased intermediatefrequency signal from receive signals of transducer arrays withdifferent operating frequencies.
 13. The acoustic imaging system ofclaim 11 further includingmeans for summation of receive signals derivedfrom groups of adjacent transducers to produce a single signal for eachtransducer group that connects to one delay cell as (1) the signalderived from the processed receive signal of at least one transducer.14. The acoustic imaging system of claim 12 further includingmeans forsummation of receive signals derived from groups of adjacent transducersto produce one intermediate frequency signal for each transducer groupthat connects to one delay cell as (1) the signal derived from theprocessed receive signal of at least one transducer.
 15. The acousticimaging system of claims 1, 2, 3, 4 or 5 further includingphaseadjustment means to achieve phase coherence between the receive signalsof at least two adjacent transducers to permit course quantization ofthe delay elements in said plurality of delay cells; and means to changethe phase adjustment in a dynamic fashion during the reception of eachacoustic echo in order to maintain phase coherence of transducer receivesignals at different depths from the transducer array.
 16. The acousticimaging system of claims 1, 2, 3, 4 or 5 further includingtransmitapodization means to vary the amplitude of each said transmit pulseapplied to each transducer from zero to a maximum value for improvedsidelobe performance of the transmitted acoustic energy and extension ofthe depth of focus.
 17. The acoustic imaging system of claims 1, 2, 3, 4or 5 further includingdynamic receive apodization means to vary the gainof each transducer receive signal from zero to a maximum value to allowthe active receive aperture to be small by setting apodization values ofouter transducers to zero when said receive signals are from close-intargets, and said aperture to be large when receive signals are fromtargets farther from the transducer in order to improve the depth offield at all depths from which receive signals are obtained and toimprove sidelobe performance by controlling the shape of the activereceive aperture.
 18. The acoustic imaging system of claims 1, 2, 3, 4or 5 further includingdigital timing and control means to maintain anapproximately constant ratio of receive focal range, which isdynamically variable, to receive aperture size, which is alsodynamically variable, over at least a portion of the receive depth toobtain approximately constant lateral resolution over said portion ofthe receive depth.
 19. The acoustic imaging system of claims 1, 2, 3, 4or 5 further includingphase adjustment means to achieve phase coherencebetween the receive signals of at least two adjacent transducers topermit coarse quantization of the delay elements in said plurality ofdelay cells; heterodyning means in conjunction with the phase adjustmentmeans in order to derive a phased intermediate frequency signal fromreceive signals of transducer arrays with different operatingfrequencies; and means to change the phase adjustment in a dynamicfashion during the reception of each acoustic echo in order to maintainphase coherence of transducer receive signals at different depths fromthe transducer array.